This invention relates to a method for updating data in bubble cassette memories and, more particularly, to a method for updating data in a bubble memory which enables the simultaneity of data to be preserved even if a power failure should occur in the course of the updating operation.
Wire-cut electric discharge machines are well-known in the art. In such machines a wire is tensioned between an upper guide and a lower guide, and an electric discharge is produced between the wire and a workpiece placed on a table. By moving the workpiece (table) in the X and Y directions along a machining path, the workpiece is cut as instructed. When the wire is tensioned so as to lie perpendicular to the table (workpiece), the upper and lower surfaces of the workpiece can be cut to the same shape. It is also possible to adopt an arrangement in which the upper guide can be displaced in the X and Y directions, such as in a direction at right angles to the direction in which the workpiece is travelling, to incline the wire with respect to the workpiece. This causes the upper and lower surfaces of the workpiece to be cut to different shapes, enabling so-called taper cutting.
A discharge machining operation performed by the wire-cut discharge machine of the aforesaid type can continue for an extremely long period of time, such as 12 hours, 24 hours, and even for as long as one week in some cases. The workpieces which are cut by electric discharge machining generally are mold materials and the like, and some materials can be extremely expensive.
Accordingly, since an interruption in power can occur during machining, such as by inadvertently turning off the power supply or as the result of a power failure due to a lightening bolt or some other cause, it is desired that cutting be resumed, as soon as power is restored, from the position formerly occupied by the wire electrode prior to the power interruption. This prevents the workpiece from being wasted. For a long-term discharge machining operation of one day or one week, as mentioned above, machining proceeds continuously day and night. It follows, then, that there is need for a system which, in the event of a power interruption that occurs at night, is capable of resuming machining automatically without operator intervention after power is restored.
When power is interrupted, there is destruction of information indicative of the current and commanded positions of a motor or of a movable element such as the table, and destruction of positional control information relating to backlash direction, pitch error compensation dog number and the like. There is also complete destruction of the internal status of an NC, which includes interpolation control information such as the number of interpolation pulses produced prior to the power interruption as well as the block number, counting from the beginning of the numerical control data. Also, when power is interrupted, the motor or a movable element such as the table moves by a small amount on the order of several microns, by way of example. Accordingly, when power is cut off during a discharge machining operation in the prior-art arrangement, discharge machining cannot be resumed immediately, in automatic fashion without human intervention, from the position occupied before the interruption in power. Instead, it has been conventional practice to adopt the method based upon the following sequence:
(1) The cutting starting point of the discharge machining operation is stored in memory in advance. For example, this may be set in a digital switch or stored in non-volatile memory.
(2) Following the restoration of power the wire is removed and the table or wire is returned to the zero point of the machine. This brings the position of the table, which is the movable element, or of the wire into coincidence with the current position stored in a volatile memory.
(3) Following the return to the zero point, the table or wire is positioned at the cutting starting point by using the cutting starting point information stored in the non-volatile memory or set on the digital switch.
(4) Upon completion of the cutting starting point positioning operation, the wire is rethreaded and a return is effected to the beginning of the numerical control data.
(5) The operator rewinds the command tape to the beginning of the data. The table or wire is then moved along the programmed path at a speed greater than the commanded speed, starting from the beginning of the numerical control command data. This causes the table or wire to travel along the previously cut path. By way of example, a well-known dry-run function can be employed to transport the wire or table at the higher speed.
(6) When the table or wire has been moved as far as a position just short of that occupied at the time of the power interruption, the feed speed of the table is restored to that specified by the program, machining power is introduced, and an electric current is passed through the wire to resume the discharge machining operation.
With the conventional method as described above, processing following the re-introduction of power is extremely complicated and a considerable period of time is required to restore the cutting operation. The result is a marked decline in machining efficiency. Since an operator intervenes in order to resume the machining operation, moreover, an interruption in machining caused by a power failure which occurs at night or during a holiday will not be remedied until the next working day. This greatly diminishes machining efficiency.
In view of the foregoing, the Applicant has already proposed a status recovery system wherein machining is resumed automatically, and wherein the restarting of a machining operation following the re-introduction of power is accomplished in a short period of time requiring neither a return to a zero point, movement to a cutting starting point nor rethreading of the wire. This previously proposed status restoration system will now be described in brief.
As mentioned above, such information as positional control and interpolation information stored in the memory of a numerical control device is destroyed when power is interrupted, and a table or other movable member traverses a very short distance (several microns) upon the occurrence of the power interruption. Accordingly, when power is interrupted owing to a power failure or the like, machining can be resumed automatically by restoring the main power supply providing that the following conditions are satisfied: (A) power is capable of being applied to the numerical control device automatically by restoring the main power supply; (B) the movable member such as the table is capable of being returned automatically to the position occupied prior to the interruption in power; and (C) the internal status of the numerical control device is capable of being restored automatically to the status which prevailed prior to the interruption in power.
To this end, in accordance with the previously proposed system, a device is provided which, in response to restoration of the main power supply, automatically introduces electric power to the numerical control device and the like. Further, in order to automatically return the movable member such as the table to the position occupied prior to the power interruption, the positional information which prevailed prior to the power interruption is preserved in a non-volatile memory. Also provided is a position sensor capable of sensing the absolute position of the motor or of the movable member such as the table. Following the application of electric power, the movable member such as the table is positioned and returned to the position occupied prior to the power interruption by using the positional control information which prevailed prior to the power interruption, which information is stored in the non-volatile memory, and the absolute position detected by the position sensor.
Furthermore, to automatically restore the internal status of the numerical control device to what it was before the interruption in power, in accordance with the previously proposed system, the location of a block which contains the numerical control data that prevailed prior to the power interruption, as well as such interpolation information as the number of interpolation pulses in said block, are stored in the non-volatile memory, and the numerical control command data from the beginning of the data is retrieved so that numerical control processing can be re-executed starting from the retrieved numerical control data. Then, after power is restored, the motor and the movable element such as the table are locked against movement, numerical control processing is executed sequentially starting from the numerical control command data at the beginning of the program, and processing is halted temporarily when the block location and the number of interpolation pulses, which is based on the numerical control command of said block, coincide with the block location and the number of interpolation pulses, respectively, stored in the non-volatile memory. The internal status of the numerical control device is thus made to coincide with what it was immediately prior to the interruption in power. When the conditions which prevailed just prior to the power interruption are restored, the feed of machining fluid and the introduction of electric current to the wire are carried out, and discharge machining is resumed.
In accordance with the previously disclosed system as described above, therefore, excellent effects are obtained since the discharge machining operation can be resumed automatically without human intervention once power has been applied following the power interruption.
In the previously proposed system, the non-volatile memory stores the positional control information such as the current position prior to the power interruption, the block location containing the numerical control command data which prevailed at such time, as well as the interpolation information such as the interpolation pulses in said block. A bubble memory is employed as the non-volatile memory. The above-mentioned positional control information and interpolation information is successively written into the bubble memory periodically at predetermined short time intervals to update the data within the bubble memory, so that the most recent positional control information and interpolation information is stored in the non-volatile memory at all times.
The numerical control information and interpolation information stored in the bubble memory is large in quantity and cannot be written into the memory by a single command. Instead, the information is written into the memory by dividing the information into a number of portions each of which is written at a separate time. If a power failure or the like were to occur during the writing of new data into the bubble memory, therefore, part of the data in the memory would be new, and part would be old. The simultaneity of the data would therefore be destroyed, and it would not be possible to execute the correct processing to cope with the power failure.